I can only imagine what you are feeling. Please forgive yourself. Although your instincts were to take the kittens with you, others told you otherwise. These were people who you sought out, including shelters. You had no reason to disregard their advice. Your intentions were good, and it’s quite possible the mother would have returned for them. Remember hindsight is 20:20.
The shock of finding the kittens later would devastate most people, so your feelings are expected. There is no way you could have known they would perish in such a short time without intervention.
You were able to interact with one of the kittens and brought it love and comfort. That speaks volumes about your character. Please take solace in knowing that small act may have been the only human contact that kitten had in its short life.
You are obviously a very caring person with tremendous heart. I don’t care how tough you may ordinarily feel. There are just some things that will affect you in unexpected ways. You will heel from this in time, but allow yourself to grieve. Sending prayers for comfort and strength. Be well.